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Revista Brasileira de Geofı́sica (2014) 32(4): 673-679
© 2014 Sociedade Brasileira de Geofı́sica
ISSN 0102-261X
www.scielo.br/rbg
COMPARISON BETWEEN RESULTS OF SEISMIC REFRACTION
AND STANDARD PENETRATION TEST (SPT) TO STUDY SHALLOW GEOLOGICAL
SUBSURFACE IN AN URBAN AREA OF BRASÍLIA, BRAZIL
Pedro Vencovsky Nogueira, Marcelo Peres Rocha, Welitom Rodrigues Borges,
Eduardo Xavier Seimetz and Márcio Maciel Cavalcanti
ABSTRACT. The most common procedure for an engineering project/construction is the use of direct survey, borehole and Standard Penetration Test (SPT). This
provides punctual information of the geology at the site, and many boreholes are necessary along the construction site, representing a significant amount of the budget
for the construction and to help develop a better geological understand/map of the site. The use of geophysical methods allows to study the subsurface by indirect
means, with low cost, and enable to cover large areas if compared to direct surveys. Geophysical methods are increasingly being used in engineering works, however, in
Brazil the use in engineering projects is still scarce. In this work was used shallow seismic refraction method to study the shallow subsurface in an area along the future
track of the subway system of Brası́lia, Brazil. The refraction results (P-wave) were compared with previous existing data from Standard Penetration Test (SPT), and soil
profile description. The seismic was used to study the subsurface geology, and SPT data were used to compare the seismic results. We observed a good correlation
for the depths obtained through each method, mostly in the north portion of the line, when the SPT was near the line, indicating that its results are influenced by the
same mechanical parameters, related to soil strength. Our results motivate the use of seismic refraction as a tool to optimize the direct investigation methods for better
geotechnical characterization of the medium.
Keywords: shallow seismic refraction, standard penetration test (SPT), geotechnical study.
RESUMO. O procedimento inicial mais comum em um projeto de engenharia é o uso de pesquisa direta, por meio de sondagens e Índice de Resistência à Penetração (SPT, em inglês). Estas ferramentas fornecem informações pontuais acerca da geologia local, sendo necessárias diversas sondagens para desenvolver um bom
entendimento geológico/geotécnico da região, fazendo com que as sondagens representem uma quantidade significativa do orçamento da obra de engenharia. O uso
de métodos geofı́sicos permite estudar a subsuperfı́cie por meio indireto, com baixo custo, e possibilita cobrir grandes áreas, quando comparado ao uso exclusivo de
sondagens diretas. Métodos geofı́sicos estão sendo cada vez mais utilizados em obras de engenharia, no entanto, o seu uso em projetos de engenharia no Brasil ainda
é escasso. Neste trabalho foi utilizado o método de sı́smica de refração rasa para estudar a subsuperfı́cie em uma área ao longo do futuro trecho do sistema de metrô de
Brası́lia, Brasil. Os resultados de refração (onda P) foram comparados com os dados pré-existentes de SPT e descrição do solo. A sı́smica foi empregada para estudar
a geologia da subsuperfı́cie, os dados SPT foram utilizados para comparar com os resultados sı́smicos. Observou-se uma boa correlação para as profundezas obtidas
através de cada método, principalmente na porção norte da linha, região em que o SPT está mais próximo da linha, indicando que os seus resultados são influenciados
pelos mesmos parâmetros mecânicos, relacionados com a resistência do solo. Nossos resultados motivam o uso de refração sı́smica como uma ferramenta para
aperfeiçoar os métodos de investigação direta, com objetivo de gerar uma melhor caracterização geotécnica do meio.
Palavras-chave: sı́smica de refração rasa, ı́ndice de resistência à penetração, estudo geotécnico.
Universidade de Brası́lia, Campus Universitário Darcy Ribeiro, Institute of Geosciences – ICC, 70910-900 Brası́lia, DF, Brazil.
Phones: +55(61) 8123-0671; +55(61) 9952-6668; +55(61) 9646-9090; +55(61) 9609-8492; +55(61) 8177-7417
– E-mails: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]
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INTRODUCTION
In the beginning of engineering works the knowledge and the
conditions of the subsurface is crucial because the geological
structures can be a complex factor. A typical approach is to study
the subsurface in a direct way, using boreholes and Standard
Penetration Test (SPT) (Hettiarachchi & Brown, 2009). Generally, direct survey provides high quality information, however,
represent punctual information. Several surveys are necessary
to have a greater understand of the geology, increasing the cost
of the work. Direct surveys are “invasive techniques”, where the
study area can be permanently affected, and at some cases, limit
the application at urban areas (McDowell et al., 2002). Several
methods, such as resistivity, ground penetrating radar (GPR) and
seismic can be used combined with direct geotechnical surveys
to reduce its quantity and cost (McDowell et al., 2002; Consenza
et al., 2006).
The main characteristic of geophysical methods is to provide
information by indirect means, covering both large and small
areas, and in different scales. The cost of geophysical surveys
is considerably smaller when compared to the exclusive use of
direct survey, usually spending less time to be done.
The SPT is a common method used in engineering projects
to define soil strength. The number of blows needed per depth for
the STP can be related to vertical resistance to ground penetration (Yagiz, 2008; Alves, 2009). Several authors use both seismic
refraction and SPT to define soil conditions such as, type of soil,
rippability and liquefaction (Basarir & Karpuz, 2004; Basarir et
al., 2008). Generally, SPT data is compared with shear wave (Vs )
measurements, providing better results, since shear wave propagation is not affected by the water content of the medium
(Hasancebi & Ulusay, 2007; Anbazhagan & Sitharam, 2008).
However, in this work, due to equipment restrictions, the SPT data
was compared with primary wave (P-wave).
The seismic refraction is one of the main geophysical techniques used in geotechnical problems (e.g. Khalil & Hanafy,
2008). Among the most common applications of seismic refraction in a geotechnical context is to measure the depth of the
bedrock, for the construction of large and tall buildings, dams
and highways (Telford et al., 1990). There are some examples of
the use of seismic refraction in Brazil (Prado, 2000; Martı́nez &
Mendoza, 2011). For more information about application of geophysical methods in Brası́lia, see Seimetz (2012).
In this work, we used shallow seismic refraction to study the
subsurface for a future subway station in Brası́lia, Brazil (Fig. 1).
The main objective was to generate a geological model based
on shallow seismic refraction results, in order to understand the
shallow geological structure in the study area and also compare
the geological model, seismic results, with the Standard Penetration Test (SPT) data.
METODOLOGY
For the data acquisition of shallow seismic refraction, were used
48 receivers spaced 2 meters apart (Fig. 2). The seismic source
used was a hammer with 8 kg, struck 15 times against a metal
plate placed on the ground. A total of six seismic sections were
acquired, all with 94 meters in length (Fig. 3), composing a seismic line with 564 meters (Figs. 1 and 3). The positions of the
source for each line were at: –2, 47 and 94 meters, from the
first geophone. The data was acquired by using the Geode (Geometrics) seismograph and for the data processing was used the
program package SEISIMAGER 2D. The technique used to process the data was the time-term inversion.
The SPT is a classic geotechnical method used to measure
mechanical properties of the medium. A sample tube is driven
into the ground using a standard weight, which is dropped freely
from a standard height. According to international standards
(ASTM, 2008), the number of blows required for this tube to
penetrate 15 cm into the soil is recorded and resulting a graph
of blows quantity with respect to depth. This graph can be related to variations in the strength of the material in the subsurface,
since the blow counts are related to density of the ground (Brown
& Hettiarachchi, 2008; Murley & Hettiarachchi, 2011). For the
region of Brası́lia, when the SPT blow count is higher than 50,
the material is considered impenetrable. Meaning that, from this
depth on, the soil provides good ground stability for engineering purposes (Alves, 2009). This was the definition used in this
work to underline the depth of the impenetrable. We selected the
eight SPT probing most close to the seismic line. These eight SPT
data where separated in three groups, according to soil description and depth of the impenetrable level. Group 1 includes SPTs
numbers 935, 938 and 940 (yellow circles in Fig. 1), Group 2
contains SPT numbers 952 and 953 (orange circle in Fig. 1).
Group 3 includes SPTs 985, 986 and 993 (Red circles in Fig. 1).
The SPT data used in this work were acquired by engineering
companies with the supervision of the Department of Civil Engineering (ENC) of the Universidade de Brası́lia (UnB).
RESULTS
Figure 2 shows an example of a seismogram obtained by the
method of shallow seismic refraction. In the y-axis is the time,
and the x-axis represents the geophone offsets. It is observed
that for the geophones more distant from the shot point (where
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Figure 1 – Study area on the Asa Norte region of Brası́lia, between the blocks 112 and 113. The red line is the seismic profile. The SPT data (color circles) were
separated by groups (different colors) related to their distance from the seismic line, depth of the impenetrable layer (from SPT results) and soil description.
Figure 2 – Example of seismograms acquired in this study. In this case, the source position was –2 meters related to the first geophone. The purple line represents
the marking of first breaks.
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Table 1 – Summary of information obtained with the methods of seismicrefraction and SPT.
SPT
number
Position on
seismic line (m)
Distance from
seismic line (m)
Thickness –
seismic (m)
Thickness –
SPT (m)
935
938
940
952
953
986
985
993
17
47
66
155
179
541
543
557
105
106
112
106
106
267
253
229
3,5
4
5
7,5
7,5
11
11
10
4
6
6
9
8
22
23
21
the arrival of the wave occurs later) the signal/noise ratio is lower.
This is probably due to the decrease of the seismic signal energy far from the source, which makes the record more susceptible to the effects of pedestrian circulation nearby the geophones,
cars passing on roads nearby and the influence of trees when the
weather started to get strong winds at the study site. However, it
was possible to identify the first arrivals at all geophones.
The comparison between the results obtained from seismic
refraction and SPT are shown in Figure 3. A summary of information obtained from these results is presented in Table 1.
Six velocity models (one for each sub-section) were generated
for the study area (Fig. 3B), where two layers were observed with
different velocities, according to the time-distance curves generated from the picking of the first arrivals (Fig. 3C). The velocities
obtained for the first layer varies from 402 to 540 m/s, and the
velocity for the second layer varies from 1519 to 1791 m/s.
SPT data and soil description were compared to parts of the
seismic line (Fig. 3A). The SPT graph is represented in yellow
lines, where the abscissa represents the number of blows (from
0 to 60 blows) and the ordinate axis represents the depth. The
soil description is represented by the square blocks. Their composition varies from embankment, originated from construction
works, clay and siltstone.
DISCUSSION
The seismic line was separated in three different regions: North,
Center and South regions, according to soil description data, NSPT values, seismic velocities data and the depth of the seismic
refraction interface. The North, Center and South regions are related to SPT groups 1, 2 and 3, respectively (Fig. 1). Each region
was correlated with a different seismic section; the North region
includes only section 1. The Center region of the line includes
sections 2, 3, 4 and 5, and the South region is related with section 6. This division was mostly based on the depth of the seismic
interface and the velocity of the second layer. The velocity of the
first layer was not considered as a factor for this division because
can be highly affected by engineering works, as adding embankment or removing original soil.
In all of the three regions, we assume that the depth of the
impenetrable SPT level is related to the interface between the
two layers in the seismic model, since both methods rely on soil
strength (Basarir & Karpuz, 2004; Basarir et al., 2008). The comparison between the seismic section and SPT data from group 3
is more susceptible to mistakes because of the distance between them (260 m).
In the North region, the soil has a first layer of about 2,5 to
3 m of embankment, followed by a layer of clays until the depth of
about 10 m. There is no presence of siltstone. The SPT impenetrable level depth is shallow, about 4 to 6 m, suggesting that the
clay is responsible for the impenetrable level. Section 1 of seismic, showed that the velocity for the first layer (433 m/s) is related
to embankment, and the velocity of the second layer (1519 m/s)
is related to clay. The second layer has the lowest velocity in all
of the six seismic sections. The depth of the seismic interface
showed good correlation to the depth of the impenetrable level
of SPT data.
The soil description of the Center region showed no layer of
embankment, the first layer is associated with clays, with a thickness of 6 m, followed by a layer of siltstone until a depth of 13 m.
The SPT impenetrable level was found at a depth of 8 m, within
the layer of siltstone. In this case, it was not the change of material from clay to siltstone that resulted in the impenetrable layer,
it was the strength of the siltstone layer that increased in depth.
The velocity of the first layer (402 to 524 m/s, in sections 1 to
5) is related to clay and siltstone, the velocity of the second layer
(1620 m/s to 1791 m/s) is related to siltstone. The seismic interface was found at depth of about 7,5 m, similar to the depth found
with the SPT method.
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Figure 3 – Comparison between results of: A) SPT, soil description and seismic refraction. B) The six seismic sections together, with the velocity of each layer from each section in m/sec.
C) Travel-Time curves, obtained through the marking of the first arrivals on seismograms.
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The South portion of the line is where the correlations between seismic and SPT does not match, because of the distance
from the seismic line (260 m), and different geotechnical context.
The soil description shows a layer of embankment of about 5 m,
followed by clay and siltstone. The interface between the clay and
siltstone varies from 14 to 19 m deep. The SPT impenetrable level
was found at a depth of about 22 m, in the siltstone layer. The first
seismic layer (velocity of 540 m/s) is related to the embankment
and clay layers, and the second seismic layer (1580 m/s) is associated to siltstone. The seismic interface seems to relate with
change in soil strength of the siltstone layer. Although there is
great difference between the depths found with the SPT (22 m)
and seismic (11 m), both method show increase in the depth
from north to south.
The analysis of the SPT data and the seismic results has
shown that neither of the methods could be used exclusively to
define layers in terms of their material composition. On most of
the SPT data, the impenetrable layer was not related to direct
change in composition, but change in soil strength, defined by
factors such as density, compaction and porosity.
CONCLUSIONS
For the study area there was a good correlation between the results of seismic refraction and SPT, which showed an increase in
thickness of the shallow layer toward the South, although STP
from group 3 could not be directly correlated.
The seismic model showed that the depth of the interface between the two layers is about 4 m in the northern part of the line,
increasing to 11 m in the south. The depth of the seismic model
is compatible with the SPT data in the center and north portion of
the line.
For the study area, the correlation between seismic and SPT
showed that for seismic velocities greater than 1500 m/s, the SPT
blow count is higher than 50, outlining the impenetrable strata.
As expected, the variations in the velocity of seismic waves
should be related to variations in material resistance as observed
with the data from SPT, since seismic refraction is an efficient
geophysical method to determine soil compactness (Sturaro et al.,
2012). Furthermore, seismic results and SPT do not relate exclusively on lithology, but other factors such as density, compaction
and porosity.
These results provide arguments to increase the joint employment of geophysical and geotechnical methods, following
the examples of Fonseca et al. (2006) and Sudha et al. (2009),
specially for bigger engineering projects.
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Recebido em 22 setembro, 2013 / Aceito em 26 setembro, 2014
Received on September 22, 2013 / Accepted on September 26, 2014
NOTES ABOUT THE AUTHORS
Pedro Vencovsky Nogueira. Geologist graduated from the Universidade de Brası́lia and Master in Applied Geosciences, area of concentration in Applied Geophysics from the same university. Has worked on projects regarding with shallow geology/geophysics, focusing on seismic refraction, resistivity, induced polarization
and ground penetrating radar.
Marcelo Peres Rocha. Graduated in Full Degree in Physics from the Universidade Federal de Mato Grosso (2000), Master of Science (Geophysics) from the
Universidade de São Paulo (2003) and Doctor of Science (Geophysics) from the Universidade de São Paulo (2008) with 6 months stage during the year 2006 at the
Institute of Earth Sciences Jaume Almera in Barcelona, Spain. Has experience in Geosciences with an emphasis on Geophysics, working mainly on Seismology and
Earthquake. Currently, is an Adjunct Professor at the Universidade de Brası́lia and teaches classes in undergraduate courses of Geophysics and Geology, as well as
post-graduation at the Institute of Geosciences.
Welitom Rodrigues Borges. Graduated in Geology from the Universidade Federal do Mato Grosso (2000), completed master’s degree (2002) and doctorate
(2007) in geophysics from the Universidade de São Paulo. Consultant in processing GPR data in agreement between the company and Unicamp Brain Technology
(2004-2006). Geophysical consultant in the company Geopesquisa Geological Investigations (2006-2007). Consultant at GPR company SIGEO – Integrated Solutions
in Geotechnology (2005-2007). Professor of Geophysics in the Undergraduate Geology (CCA/UFES – 01/2009 the 05/2009). Currently, Professor of Geophysics at
the Institute of Geosciences, the Universidade de Brası́lia (IG/UnB). Coordinated the team Geology/Geophysics Tocantins Working Group, in search of traces of the
missing event known as the Araguaia guerrilla movement in the years 2009 and 2010. Since 2011 operates in the Brazilian Geophysical Society (SBGf) as Secretary of
the Midwest Regional. Tutor Phygeo (Junior Company of Geophysics) since its inception in 2010.
Eduardo Xavier Seimetz. Graduated in Physics from the Universidade Católica de Brası́lia (2006). Has experience in Physics, with emphasis on General Physics.
Completed the graduate program at the Institute of Geosciences in applied geosciences at the Universidade de Brası́lia – UnB. Experience with geophysical methods:
shallow seismic refraction and electrical resistivity.
Márcio Maciel Cavalcanti. Doctorate in Applied Geosciences (UnB/2013), Master in Applied Geosciences (UnB/2013), BS in Environmental Engineering (Universidade Católica de Brası́lia/2008). Practice in Applied Geophysics, methods and Electrical Resistivity Ground Penetrating Radar (GPR) and electromagnetic.
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